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Direct and simultaneous determination of copper, manganese and molybdenum in seawater with a multi-element electrothermal atomic absorption spectrometer Chien-Ly Chen, K. Suresh Kumar Danadurai and Shang-Da Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan

Downloaded on 07 January 2011 Published on http://pubs.rsc.org | doi:10.1039/B008507N

Received 23rd October 2000, Accepted 19th January 2001 First published as an Advance Article on the web 5th March 2001

A simple method is described for the determination of trace metals (Cu, Mn and Mo) in seawater directly and simultaneously using a multi-element electrothermal atomic absorption spectrometer equipped with a transversely heated graphite atomizer. A mixture of Pd(NO3)2 and Mg(NO3)2 with the special gas (5% H2 and 95% Ar) was used as the chemical modifier. Matrix interferences were removed completely, so a simple calibration curve method could be used. The relative standard deviations (RSD) for the simultaneous determination of Cu, Mn and Mo in CASS-3 (near shore seawater) were 2–12% and the detection limits were 0.42 mg l21 for Cu, 0.68 mg l21 for Mn and 1.2 mg l21 for Mo. The accuracy of the method was confirmed by the analysis of certified reference saline waters (CASS-3, SLEW-2 and NASS-5). The analytical results were compared with those obtained using pure Ar as the purge gas. Scanning electron micrographs were used to investigate the physical form of Pd on the surface of the platform in the graphite furnace.

Introduction Trace metals in seawater have been found to be difficult to analyze because of the high salt content and minute concentrations of trace metals. However, the direct determination of trace metals in seawater is of interest, because it permits simple and rapid analysis. Electrothermal atomic absorption spectrometry (ETAAS) is a powerful analytical technique for the determination of trace elements in biological and environmental materials due to its inherently high sensitivity and specificity. A wide variety of chemical modifiers have been used in the analysis of seawater by ETAAS.1 Schlemmer and Welz reported2 that the use of Pd in combination with magnesium nitrate was a fairly universally applicable modifier in ETAAS, and therefore recommended it for multi-element analysis. An advantage of using Pd-containing chemical modifiers is that high-purity reagents are available. Palladium metal acts as the chemical modifier, although it normally is introduced into the furnace as a chloride or nitrate salt. Palladium metal is obtained either through thermal decomposition in the course of the pyrolysis stage or reduction of the palladium with reducing agents such as ascorbic acid, hydroxylamine hydrochloride or hydrogen.3–5 Hydrogen is recommended for the reduction of Pd6 since it is free from the contamination, which often arises during the addition of aqueous reducing agents, and also it promotes the formation of palladium metal earlier in the temperature program. Welz et al.3 eliminated chloride interference in ETAAS determination of thallium in seawater by use of a mixture of Pd(NO3)2 and Mg(NO3)2 as the modifier and 5% H2 in Ar as the purge gas. This is the only report that describes the use of 5% H2 in Ar to analyze trace metals in seawater. No certified value of Tl was provided for the seawater sample (NASS-1) they analyzed, but pyrolysis of the modifier prior to sample deposition and use of 5% hydrogen as the purge gas gave around 100% recovery. Creed et al.4 minimized chloride interferences, produced by combining acid digestion, by using Pd/Mg(NO3)2 and 5% H2 in Ar as a modifier in ETAAS. The direct determination of Cu,7–10 Mn8,11–16 and Mo11,12,17–20 in seawater by ETAAS has been investigated extensively. Huang 404

and Shih7 determined Cu in seawater directly utilizing ammonium nitrate as the chemical modifier. The detection limit was 0.3–0.4 mg l21 (20 ml), which was further decreased to 0.07 mg l21 by using multiple injections (5620 ml). Carnrick et al.16 compared the results obtained in the direct determination of Mn in seawater using Zeeman background correction and deuterium background correction. The detection limits were 0.1 mg l21 for single injection (20 ml) and 0.02 mg l21 for multiple injections (3625 ml). A recent publication by Chan and Huang10 reported the direct determination of copper and cadmium in seawater using ammonium nitrate for Cu and a mixture of diammonium hydrogen phosphate and ammonium nitrate for Cd as chemical modifiers. The LOD obtained were 0.06 and 0.005 mg l21 for copper and cadmium, respectively. The direct determination of Mo in seawater by ETAAS has been reviewed by Huang et al.,13 who determined Mo using various chemical modifiers. The use of Pd(NO3)2 as the chemical modifier offered a better detection limit (0.29 mg l21) than the use of Mg(NO3)2, ascorbic acid or no chemical modifier. Tominaga et al.19 determined molybdenum in coastal seawater using ascorbic acid as the chemical modifier. The detection limit was 6 mg l21 and relative standard deviations were within 12%. Grobenski et al.12 determined molybdenum in a seawater reference standard (NASS-1) using Zeeman ETAAS without modifier. The value found was in excellent agreement with the certified value; the RSD value was 15%. Nakahara and Chakrabarti20 determined molybdenum in synthetic seawater (Mo, 50 mg l21) without a chemical modifier; the average RSD was better than 10%. According to these works12,19,20 it appears that direct determination of molybdenum in seawater with ETAAS is possible, but improved precision of the determination may be required. Only two papers have reported on the simultaneous and direct determination of trace metals in seawater by ETAAS.21,22 Su and Huang21 determined Cu and Mn in seawater simultaneously with detection limits of 0.07 mg l21 for Cu and 0.10 mg 121 for Mn. They determined Mo and V in seawater simultaneously with detection limits of 0.35 mg l21 for Mo and 0.32 mg l21 for V.22

J. Anal. At. Spectrom., 2001, 16, 404–408 This journal is # The Royal Society of Chemistry 2001

DOI: 10.1039/b008507n

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This is the first attempt at the direct and simultaneous determination of three trace elements in seawater. The aim of this study is to determine Cu, Mn and Mo in seawater directly and simultaneously with a multi-element ETAAS (SIMAA 6000). 5% H2 in Ar along with a mixture of Pd and Mg is used as the chemical modifier. The absorbance characteristics with and without the addition of hydrogen are compared and scanning electron micrographs are presented to show the distribution of palladium in the graphite furnace during the heating cycle. We found that matrix interference could be removed completely using 5% H2 in Ar, so that a calibration curve method could be applied. The detection limits were 0.42, 0.68 and 1.22 mg l21 for Cu, Mn and Mo, respectively. The accuracy of the developed method is confirmed by the analysis of three certified reference saline waters (CASS-3, NASS-5 and SLEW-2).

Experimental


Instrumentation All tests were performed on a multi-element ETAAS system (Model SIMAA 6000, Perkin-Elmer, Norwalk, CT, USA) with a transversely heated graphite atomizer (THGA), a longitudinal Zeeman-effect background corrector and an autosampler (Model AS-72). A pyrolytic graphite-coated THGA graphite tube with integrated platforms was used. The rate of flow of the normal gas (100% Ar) or special gas (5% H2z95% Ar) was 250 ml min21. Stopped flow during the atomization was used. The procedure was controlled by AA Winlab software version 2.3 (Perkin-Elmer, Norwalk, Connecticut, USA). The lamps used were hollow cathode lamps (HCL) from Perkin-Elmer and the wavelengths used were: Cu 324.8 nm, Mn 279.5 nm and Mo 313.3 nm. The sample injection volume was 20 ml. Unless otherwise specified, each experimental datum is the arithmetic average of three determinations. Peak area of the atomic absorption signal was used for the determination. Surface studies of the platform were performed using a scanning electron microscope, JEOL, JSM-840. Reagents and samples High-purity water (18 MV cm) was prepared with a deionized water system (Milli Q, Millipore Corp.). Nitric acid (Merck suprapure grade) was purified by subboiling distillation. Commercial copper, manganese and molybdenum standards (1000 mg l21, Merck) were used. A solution of Pd(NO3)2 was prepared by dissolving 100 mg of palladium metal powder (Merck) in 1 ml of concentrated nitric acid and diluting to 100 ml with deionized water. If dissolution was incomplete, 10 ml of suprapure grade concentrated hydrochloric acid were added to the cold nitric acid solution, which was then heated gently until the solution cleared. The solution was then heated to gentle boiling in order to volatilize the excess chloride.3 A solution of Mg(NO3)2 (15% HNO3, 10 000 mg l21, AA reagent grade) was obtained from KANTO Chemical, Japan. The Mg(NO3)2 solution (10 000 mg l21) was diluted to the desired concentration with HNO3 solution (15% v/v). Seawater reference materials, CASS-3 (nearshore seawater), SLEW-2 (estuarine water) and NASS-5 (open ocean seawater) were obtained from the Marine Analytical Chemistry Standards Program of the National Research Council of Canada.

placed in a PTFE beaker with nitric acid (2 M, 30 ml) overnight; then the resin was transferred to a column. The resin was washed with HNO3 (2 M, 30 ml) and deionized water (50 ml). Aqueous ammonia (2 M, 20 ml) was added to convert the resin from the hydrogen form to the ammonium form. Finally, the resin was washed with deionized water (40 ml). Contamination control All reagents were prepared in a class-100 laminar flow clean hood. All sample containers, autosampler cups, etc. were acid washed with 50% v/v nitric acid for 24 h and then rinsed five times with deionized water before use.

Results and discussion Optimum conditions using a mixture of Pd and Mg as chemical modifier with special gas (5% H2z95% Ar) The reference seawater sample CASS-3 (20 ml) with added Cu (20 mg l21), Mn (10 mg l21) and Mo (10 mg l21) was used to find the optimum temperature program. Effect of ashing temperature The absorption and background absorption signals were studied at an atomization temperature of 2400 ‡C, with modification using a mixture of Pd (3 mg) and Mg (3 mg), and with a purge gas of 5% H2 in 95% Ar. The atomic absorption signals remained approximately constant as the ashing temperature varied from 1000 to 1200 ‡C and 1000 to 1300 ‡C for Cu and Mn, respectively. When the temperature increased above 1300 ‡C, the absorption signal randomly decreased for Cu and Mn due to the volatilization of these elements. The atomic signal for Mo was almost constant from 1250 to 1450 ‡C and then gradually decreased. In the temperature range of 1250 to 1350 ‡C the sensitivity was found to be high for both Mn and Mo. The background decreased to below 0.1. However, for Cu the optimum ashing temperature was between 1200 and 1250 ‡C. In order to determine all the three elements together, 1250 ‡C was chosen as an optimum ashing temperature. Effect of atomization temperature The effects of atomization temperature on the atomic absorbance and background absorbance are shown in Fig. 1. For Cu and Mn the atomic absorption signal decreased with increasing atomization temperature and the best precision was achieved at above 2300 ‡C with a lower background value.

Seawater blank preparation The seawater blank was prepared by adjusting the pH of the seawater sample to 5.0 and then passing it through an ionexchange column (Chelex-100, Bio-Rad Laboratory, sodium form, 100-200 mesh) at a flow rate of 0.2–0.3 ml min21. The procedure for purifying this resin (Chelex-100) was similar to that used by Sturgeon et al.23 The resin (Chelex-100, 4 g) was

Fig. 1 Influence of atomization temperature on atomic and background absorption signals at ashing temperature 1250 ‡C. Symbols are: & atomic and % background for Cu; $ atomic and # background for Mn; , atomic and + background for Mo. Modification with a mixture of Pd (3 mg) and Mg (3 mg) using purge gas 5% H2 in 95% Ar.

J. Anal. At. Spectrom., 2001, 16, 404–408

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However, for Mo, the absorption signal increased with increasing atomization temperature and had its maximum value in the temperature range 2450–2500 ‡C. Generally the Mo concentration was higher than those of Cu and Mn in seawater. Hence we decided to choose 2400 ‡C as the optimum atomization temperature for all the three elements.


Effect of Pd(NO3)2 The effect of Pd(NO3)2 modifier concentration on the atomic signal and background absorption is shown in Fig. 2. The concentration of Mg(NO3)2 was kept as 3 mg. For Cu, the absorption and background signals remained approximately constant on addition of Pd (3–9 mg). It has been reported24 that, during furnace heating, the analyte dissolves in molten palladium and may combine with it chemically. Schlemmer and Welz2 reported that most of the analytes are stabilized to about 1200 ‡C by palladium. Therefore at an ashing temperature of 1250 ‡C the matrix was removed efficiently without the loss of analyte. A range of 3–9 mg of Pd is best for Cu, and at 3 mg the background is lower than at other concentrations. Hence, 3 mg of Pd was chosen as the optimum for Cu. A similar trend was observed for Mn. In the case of Mo, the atomic absorption signal decreased as the Pd concentration increased. This kind of problem was described by Frech et al.25 By increasing the Pd mass, the time taken to completely volatilize Mo will increase, and will be evident from the delayed and decreased peak maximum value and more pronounced tailing. For multielement analysis, 3 mg of Pd was used as it produced results with high absorption signals and better precision. Effect of Mg(NO3)2 The absorption signals obtained by varying the Mg(NO3)2 concentration are shown in Fig. 3. The addition of magnesium facilitates the uniform distribution of palladium over the graphite platform surface. Smaller molten droplets of palladium are formed and, hence, sharper absorbance peaks are produced.26 When the dosage of Pd was kept at 3 mg, the optimum dosages of Mg were 3–7 mg for Cu and Mn. Mn showed maximum absorbance at 3 mg of Mg, but the precision was low. Above 7 mg the background increased. Therefore, for Mn, 5 mg of Mg is the optimum dosage. For Mo, as the concentration of modifier increased, the sensitivity decreased. For measuring all three elements, the optimum concentration of Mg was chosen to be 5 mg.

Fig. 3 Influence of magnesium on atomic and background absorption signals. Symbols as in Fig. 1; modification Pd (3 mg) and purge gas 5% H2 in 95% Ar. Ashing temperature 1250 ‡C; and atomization temperatures 2400 ‡C.

temperature was 1200 ‡C and 5ug of Pd was chosen as the maximum. For the determination of Cu, the background absorption was significantly higher than when using the special gas (0.1 compared to 0.05). This is presumably due to the fact that H2 gas eliminates the chloride interference from NaCl in seawater.4,27 Scanning electron microscopic examination of palladium distribution on L’vov platforms In order to investigate the effect of hydrogen on palladium, the scanning electron microscope (SEM) was used to examine the distribution of Pd on the graphite platform. Palladium (21 mg), Mg (20 mg) and CASS-3 (9 ml) were pipetted into a platform in the graphite furnace, dried, and then heated to an ashing temperature of 1250 ‡C. After cooling, the platform was carefully transferred to the SEM. Figs. 4 and 5 are electron micrographs taken of platforms

Optimum conditions using a mixture of Pd and Mg as chemical modifier with normal gas (100% Ar) The optimum conditions with normal gas were similar to those with the special gas (5% H2) except that the optimum ashing

Fig. 2 Influence of palladium on atomic and background absorption signals. Symbols as in Fig. 1; modification with Mg (3 mg) and purge gas 5% H2 in 95% Ar. Ashing temperature 1250 ‡C; and atomization temperatures 2400 ‡C.

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Fig. 4 Scanning electron micrographs of Pd (21 mg), Mg (20 mg) and CASS-3 (9 ml) deposited on pyrolytic graphite platform; graphite heated to 1250 ‡C with 5% H2 in 95% Ar. (a) 10 000 times magnification and (b) 30 000 times magnification.

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Table 3 Detection limits (DL) (calculated as three times the standard deviation of seven replicate measurements of a seawater blank), relative standard deviations (RSD) and recovery obtained for the simultaneous determination of Cu, Mn and Mo in CASS-3 seawater

Cu Mn Mo

DL/mg l21

RSD (%)

Recovery (%)a

0.42 0.68 1.22

12.4 7.9 2.0

112.5 100.6 94.2

a

Recovery of spiked Cu (0.5 mg l21), Mn (2.5 mg l21) and Mo (10 mg l21).

absence of H2, the Pd particles were larger, which makes diffusion difficult for the analyte, as evidenced by the fact that broader peaks were observed. The Pd particle size was 0.15– 0.4 mm when the purge gas contained 5% H2. It was larger (0.3 to 1 mm) when 100% Ar was used. This is due to the faster reduction of Pd by hydrogen and the maintenance of the Pd in its reduced form.


Analysis of certified reference seawaters

Fig. 5 Scanning electron micrographs of Pd (21 mg), Mg (20 mg) and CASS-3 (9 ml) deposited on pyrolytic graphite platform; graphite heated to 1250 ‡C with 100% Ar. (a) 10 000 times magnification and (b) 30 000 times magnification.

heated with the special gas (5% H2z95% Ar) and normal gas (100% Ar). When H2 was used, the Pd particles were found to be smaller and distributed uniformly on the surface. In the Table 1 Optimum temperature program for simultaneous determination of Cu, Mn and Mo Step

Gas flow/ Gas Temperature/ o C Ramp/s Hold/s ml min21 typea Read

Drying

110 130 Ashing 1250b Cooling 20 Atomization 2400 Clean-out 2500 20 2500

1 15 30 1 0 1 1 1

30 30 40 10 7 5 5 5

250 250 250 250 0 250 250 250

S S S S S N N N

Read

a

Gas type: S,5% hydrogen in 95% Ar; N, 100% Ar. bAshing temperature was 1200 ‡C for the runs using pure Ar.

Table 2 Results of simultaneous determination of Cu, Mn and Mo in reference seawater Sample SLEW-2 CASS-3 CASS-3 Purge gases 5% H2 in Ar 5% H2 in Ar 100%Ara

NASS-5 5% H2 in Ar

Cu found 1.67¡0.27 Cu certified 1.62¡0.11

0.551¡0.059 0.269¡0.029 0.205¡0.133 0.517¡0.062 0.517¡0.062 0.297¡0.046

Mn found 16.1¡0.4 Mn certified 17.1¡1.1

2.51¡0.02 2.51¡0.36

2.63¡0.07 2.51¡0.36

0.875¡0.044 0.919¡0.057

Mo found 4.1¡0.12 Mo certified 3.7b

8.72¡0.17 8.95¡0.26

8.74¡1.03 8.95¡0.26

10.5¡0.7 9.6¡1.0

a

Information value only. bAshing temperature 1200 ‡C.

The optimum temperature program is summarized in Table 1. Certified reference saline waters, CASS-3, SLEW-2 and NASS5 were used to confirm the accuracy of the developed method using the special gas (shown in Table 2). The analyzed values for all these elements in all saline waters were within the range of certified values, except for the case of Cu in NASS-5; the analyzed value of Cu in NASS-5 was lower than the certified value. This is because the concentration of Cu in NASS-5 seawater is below the detection limit and the absorbance is very low (0.0020). Hence copper in NASS-5 cannot be measured very accurately by this method. CASS-3 was used to compare the accuracy of the developed methods using normal gas and special gas. For Mn and Mo the found values were within the range of the certified value for both methods. The precision obtained using 5% H2 in Ar was much better than that obtained using 100% Ar. This is due to hydrogen, which improved Pd modification by forming a larger number of small palladium particles, which resulted in sharper peaks. The background was decreased and the chloride interference was possibly reduced when 5% H2 gas was used, which increased the reproducibility and calibration. For a similar reason the found value of Cu with 100% Ar was much lower than the certified result, but the found value was within the range of the certified value when 5% H2 was used. Detection limits, precision and recovery The detection limits (DL) and the relative standard deviations (RSD) using the method with 5% H2 in Ar are shown in Table 3. Detection limits were calculated as three times the standard deviation of seven replicate measurements of the seawater blank. The detection limits were 0.42, 0.68 and 1.22 mg l21 for Cu, Mn and Mo, respectively. The detection limits were higher than those of other techniques developed for single element determination. The RSD for the direct simultaneous determination of Cu, Mn and Mo in seawater (CASS-3) were in the range of 2–12.5% with a Pd and Mg mixture and 5% H2 as the chemical modifier. The precision of this technique for Mo determination was much better than the other previously developed techniques (RSD, 2% compared to 10–15%) The recoveries of spiked Cu (0.5 mg l21), Mn (2.5 mg l21) and Mo (10 mg l21) were in the range 94–112%.

Conclusion Cu, Mn and Mo in seawater can be determined directly and simultaneously by ETAAS using a mixture of Pd and Mg with J. Anal. At. Spectrom., 2001, 16, 404–408

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either special gas (5% H2 in Ar) or 100% Ar. A simple calibration curve can be used. Electron microscopic examination showed a more uniform distribution of smaller particles of Pd on the platform when a gas mixture of 5% H2 in Ar was used. The use of 5% H2 in Ar eliminated chloride interference and showed improved precision and lower background absorption. The precision of this technique for Mo determination was much better than those of other techniques developed for single element determination (RSD, 2% compared to 10– 15%). The sensitivity and accuracy of this technique are comparable to those of other techniques for single element determination. The detection limits of this work are higher than those of other techniques.

Acknowledgement We thank the National Science Council of the Republic of China for support (grant No. NSC 88-2113-M007-001).


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